2010 Prius ICE wouldn't shut off today

Forgive me if this has been posted before, but I couldn't find anyone with the same situation. Long time lurker, first time poster....

I drive a 2010 Prius with 146k miles.

Today as I was just getting to work, I noticed the engine turn on for some unknown reason as I got off the highway. There was no one behind me, so I was deliberately using the regen the entire way, so initially I had two thoughts - 1) I had charged the battery enough that the engine started to burn it off, or 2) something finally happened with my spark plugs, which have yet to be replaced (yes, I know... that needs to happen real soon). After this happened as I drove the last mile or so to work, the engine never shut off. The car had been running for about 45 minutes at that point, mostly on the highway, so the engine being cold is definitely not the cause.

Then on the drive home, the engine turned on immediately and didn't shut off for the entire drive home, even when coming to a crawl or stop. The battery for pretty much the entire drive home sat in it's "default" state (or what I presume is a default state of sorts), or about 2-3 bars below full. Just as I pulled into my apartment building's parking garage, I decided, hey, I wonder if hitting the EV button will give me some error code or something. But instead... the engine actually shut off. Using the electric engine worked just fine. When I hit the EV button again, the engine immediately turned on. I drove around the parking garage enough to determine that every time I put it into EV mode, the engine would promptly shut off, and when taken out of EV, the engine immediately turned on and wouldn't shut off.

What the heck is going on?

About the spark plugs - I haven't noticed anything weird about them yet, still getting decent mileage (just shy of 50 MPG, so not bad for winter tires and below freezing temps), and it still seems to run fine and has no shortage of power. It does grumble a little when starting up and it's really cold out, but nothing other than that.

Reading some forums, perhaps it could be the 12V battery? I know it has been changed once already, but I'd have to dig out the log to figure out when that was.

Thanks. Any thoughts or suggestions are appreciated.

 

Hi all, thanks for the replies. In response:

I don't run my heater... yes I live in Canada, but I just wear a hat and gloves. I don't need a climate controlled environment everywhere I go, plus I'm a nerd and would rather suffer in the cold to get better MPG lol.

By 'deliberately using the regen' I probably should have said 'deliberately not using the brake pads'. I know I'm always using the regen brakes when braking (except under ~7mph). I was deliberately not braking hard enough to engage the pads in order to re-coop as much charge as possible.

It could have been something with the 12V battery... although I'm not noticing much of a noticeable drop in MPG.

No signs of the HV battery going dead. Seems to increase/decrease at the normal rate.

I fail to see how long I've owned the car as being a factor in why my ICE engine wouldn't shut off, but if you must know, I have owned the car about a year now, since it had 130k on it. I have the full service records as they were done by the dealership previously, and the seller gave them to me. I also pulled a carfax when I bought it which matched the records given to me. The car was taken *very* good care of.

At any rate, I still have no idea what the issue was, however, today the ICE was turning on and off just as it's supposed to. Once it had warmed up in both morning and evening drives, it was turning off properly when not accelerating hard or at highway speeds. I was able to accelerate at the normal rate on just the electric engine while in traffic, so it seems to be operating perfectly now.

I'm not sure if just the car fully cooling down overnight (in a 55-60 F heated parking garage) reset it or if me hitting the EV button last night influenced it in some way. I'll pay closer attention to the engine over the next few days, but for right now everything seems to be fine. My best guess is the engine turned on to burn off excess charge, and for some reason didn't turn off. Perhaps the charge was still too high when I first got in the car in the evening, but that doesn't explain why it never turned off for the whole drive home (until I hit the EV button in the parking garage).